The present invention generally relates to a sealing arrangement between a rotary drum and the stationary end chamber thereof, especially a feed chamber. The function of the sealing arrangement is to prevent the access of gas from the surroundings and, on the other hand, the escape of process gas or other material, such as dust, to the surroundings in the connecting area of the rotary drum and the stationary chamber. Especially, these kinds of apparatus include rotary drum kilns and rotary drum driers, in which one end of the rotary drum is encircled by a stationary chamber for feeding of the material to be treated into the drum and the other end by a chamber for removing material.
The sealings of the ends of rotary drum driers are usually either so called labyrinth sealings or mechanical sealings having a seal ring that slides against the end of the kiln, hereinafter called mechanical sealings. The former are non-contacting sealings, the parts of which are formed in such a way that the flow resistance through the sealing is as great as possible. They are not totally tight, and thus gases and dust most often escape to the surroundings to some extent. In general, they are used when the pressure lower than the pressure of the surroundings prevails in the drum and when the conditions in the drum allow a small leakage from the surroundings into the kiln.
Better tightness is pursued by means of mechanical sealings, which are used in more demanding conditions, for example when reducing conditions prevail in the kiln and an air leakage through the sealing into the inside of the kiln would cause undesirable combustion reactions in the proximity of the leakage point.
Although having a simple structure, a labyrinth sealing is not applicable to all objects. A mechanical sealing, in turn, is more expensive and needs to be maintained significantly more often, as the slide surface needs to be lubricated in order to prevent or delay the wearing thereof. In the applications in which the dust in the kiln causes wearing, the sporadic access of dust onto the slide surfaces increases the wearing of the surfaces despite the lubrication, causing thus even a significant increase in the need of maintenance.
In causticizing plants of sulphate pulp mills, the lime is regenerated in rotary drum kilns. The recent development of the regeneration process has, however, led to situations where the area of the feed end of the kiln operates at a very high degree of feeding. Consequently, dry feed material (so called lime sludge) often gets out through the sealing and causes significant damage in the surroundings of the kiln due to excessive dusting. This happens despite the fact that in the area of the feed end of the drum at the point of the sealing there is a lower pressure inside the drum than in the surroundings. The dust leakage takes place because the particle size of lime sludge is very small, on average 10-20 xcexcm. Hereby, when being in movement, the lime sludge flows like water, penetrating the sealing.
During the operation of a rotary drum, for example a rotary drum kiln, the end of the drum does not stay in position but moves in the direction of the axis of the drum to the extent of the longitudinal thermal expansion of the drum shell. The bend of the end of the drum depends upon the load it carries, which load varies according to the degree of feeding of the drum. It also has a radial displacement, the size of which is usually, depending upon the accuracy of manufacture, a few or a few tens of millimeters. All these factors have to be taken into account in a sealing arrangement.
An object of the present invention is to create a sealing arrangement between the rotary drum and the stationary end chamber thereof arranged in a material flow relation with each other, which arrangement would be better than the known ones. The need of maintenance of such an arrangement is smaller than with sealings generally in use, such as with mechanical sealings, in which the moving and stationary parts of the sealing rub together. By means of the arrangement, the leaking of gas and solid matter (dust) is also prevented.
The invention relates to a method of preventing leakages of gas and solid matter dust by using pressurized gas, such as air, at the connecting point of a rotary drum and a stationary end chamber, such as a feed chamber, at the end of which drum there are a vertical end surface and an opening through which the drum is connected to the stationary chamber. What is essential for the method is that pressurized gas is fed on the end surface of a rotary drum toward the center axis of the drum in such a way that the angle (xcex1) between the feeding direction of the gas and the plane substantially perpendicular to the center axis of the rotary drum is 90xc2x0 or less, and in such a way that, due to the effect of the gas flow, the end surface of the drum is located at a distance from the stationary surface, whereby they do not substantially rub together in a wearing way.
The present invention also relates to a sealing arrangement defined in the appended claims.
The present invention is based on the use of pressurized gas, such as air, which is guided to the sealing in such a way that it prevents the leaking of the gas and solid matter dust from the connection point of a rotary drum kiln, and a stationary chamber, such as a feed chamber of a rotary drum kiln, and that as little air as possible is directed from the sealing out to the surroundings. Furthermore, due to the effect of the gas flow, the pressure of the gas pushes the countersurfaces of the rotary drum and the end chamber away from each other. Thanks to the air-floating brought about in this way, the countersurfaces of the rotary drum and the stationary system (feed chamber, sealing rings) do not rub together, at least not in wearing way. The advantageous aspect of the method according to the invention is that the sealing air flow required can be generated at an overall efficiency of a few kilowatts by means of a simple blower.
The sealing according to the invention operates well when said angle is 90xc2x0, but thus the required amount of sealing gas is larger than with smaller angles. The sealing gas flow typically divides in two directions, i.e. to a blow inward into the sealing and to a leakage away from the sealing. When said angle is a right angle, the sealing gas flow is divided in such a way that about half of it flows out. Not even in a case like this is a large total amount of gas required, and thus this can be regarded as a completely satisfactory arrangement. It is advantageous that the sealing does leak outward to some extent, because thereby it can be ensured that there is no excessive leakage of ambient air to the process.
The energy of the gas can be most effectively guided to prevent the penetration of the solid matter dust through the sealing of the connection point by directing the sealing air or other sealing gas obliquely, i.e. in such a way that said angle is below 90xc2x0. The oblique direction also prevents the air from passing outward from the sealing and becoming wasted air which does not contribute to the sealing process. The smaller the angle between the radius of the drum and the direction of air on the plane passing through the radius and the center axis of the drum is, the better the direction of air. The angle is below 90xc2x0, preferably about 5xc2x0 to about 60xc2x0, most preferably about 10xc2x0 to about 30xc2x0. However, the smaller the angle xcex1 is, the higher the sealing structure becomes in the direction of the radius, and also the bigger the weight and the higher the price. Hence, the size of the angle has to be optimized in each case separately. The sealing arrangement in which a significant part of the gas, for example 30% or more, flows outward from the sealing, may be quite satisfactory, in particular if it is cost-effective.
The present invention may also be applied to various apparatus comprising a rotary drum and a stationary end chamber arranged in a material flow relation with each other. A conventional apparatus included in this group is a horizontal rotary kiln which is used for calcination for example in chemical pulp and cement industry and in which the dust leakages to the surroundings of the kiln are a significant problem. These kinds of kilns are also used in other combustion processes. Drum applications are in general used in the following operations: heating/cooling of material, phase transition (e.g. evaporation of liquid), and chemical reaction (e.g. reduction and calcination). This group includes rotary driers, rotary coolers for hot material, reduction kilns, calcinators etc.